Angling | Fishing for
Bigeye Tuna

Bigeye tuna are found in tropical and subtropical waters of the Pacific, Indian and
Atlantic oceans. They are an oceanic fish, found throughout Australian waters when
surface temperatures exceed 17șC. Although juvenile bigeye tuna have not been
reported outside tropical waters.

Angling | Fishing for Big Eye
Tuna:

Bigeye Tuna have a diet which includes
squid, crustaceans, mullet, sardines, small mackerels and some deep water
species. Fishing methods are trolling deep with squid, mullet or other small
baits, or artificial lures and live bait fishing in deep waters with similar
baits.

Bigeye Tuna take trolling lures such as minnow, pusher or
squid. Trolling baits such as Mackerel, Garfish and other fish also works a
treat. Larger Bigeye can be caught using large live or whole dead bait in known
feeding areas.

The Bigeye can be found in warmer, offshore waters in
winter; migrating south during summer to cooler waters. They are able to
tolerate lower temperatures and oxygen levels than other tuna, which means they
prefer deeper water.

Bigeye tuna are believed to have recently evolved
from a common parent stock of yellowfin tuna.

One of the most difficult fish identification jobs is
distinguishing between a Bigeye Tuna (Thunnus obesus) and a Yellowfin Tuna (Thunnus
albacares).

There are two methods of examining these fish, exterior and interior. Only the
interior examination is 100 percent foolproof. The smaller the size of the tuna,
the less effective the exterior examination becomes. The easiest method is to
look for first for the characteristics of the Bigeye Tuna first, and by the
process of elimination, wind up by concluding you are holding a Yellowfin Tuna.

Exterior Characteristics of a Bigeye Tuna (Thunnus obesus):

1. The Bigeye Tuna's pectoral fins may reach to the second dorsal fin, but are
shorter than the Yellowfin Tuna's. A very short pectoral fin indicates that the
fish more than likely is a Bluefin Tuna. A very long pectoral fin that reaches
beyond the anal fin belongs to an Albacore.

2. There are 18 to 22 gill rakers on the first gill arch of a Bigeye Tuna. There
will be 24 to 28 gill rakers on a Bluefin Tuna, and 27 to 33 gill rakers on a
Yellowfin Tuna.

3. On a Bigeye Tuna, two dorsal fins are closer together and shorter than on a
Yellowfin Tuna. The forward dorsal fin of a Bigeye Tuna has 13 to 14 spines and
the rear dorsal fin has 14 to 16 rays.

4. The Bigeye Tuna has a strong lateral keel, between two small keels, located
slightly farther back on the tail, on either side of the caudal peduncle.

5. Except for the anterior corselet, the scales of a Bigeye Tuna are small.

7. The finlets of the Bigeye Tuna are bright yellow with narrow black edges.

8. Upon expiring, the body of a Bigeye Tuna loses most of its coloration,
including the disappearance of its yellow coloration. The dorsal and anal
finlets' yellow coloration turns brown after death.

9. The Bigeye Tuna's anal fin is wider and shorter than the Yellowfin Tuna's,
and it is colored all silver, with a fringe of yellow and 11 to 15 rays. In a
yellowfin tuna the anal fin is long and narrow and only silver in the middle.

10. The corselet (the front area of enlarged scales) of a Bigeye Tuna extends
further back than on a Yellowfin Tuna. These front scales are larger up front on
both tunas and become smaller along the flanks, which make these tuna appear
darker up front. These larger scales cover the fish from the first dorsal fin
down around the pectoral fins, and to the ventral fins and then takes a sharp
angle backwards. This front area of scales extends further back on a Bigeye Tuna
than on a Yellowfin Tuna, all the way to the end of the second dorsal fin. The
pectoral fin of a Bigeye Tuna will not cover this extension of scales, while on
a Yellowfin Tuna these scales extend barely as far as the second dorsal fin and
the pectoral fin will completely cover these scales.

11. The tail of a Bigeye Tuna does not have a white trailing edge. An Albacore's
tail has a white trailing edge.

12. The white spots prevalent in small Bigeye Tuna elongate as the fish becomes
older.

Interior Characteristics of a Bigeye Tuna (Thunnus obesus):

1. Without a doubt, positive identification of a Bigeye Tuna can be completed by
examining its liver. The Bigeye Tuna has a grooved liver with clearly visible
striations (fine lines looking like streaks), or dark blood vessels on the
margins of the lobes. There are two lobes that are both of the same size.
However in the yellowfin tuna these smooth lobes are devoid of these striations
and the right lobe is clearly longer than the left lobe or the middle lobe.